For thousands of years, people believed the Sun and all the heavens orbited around the
Earth.

In the sixteenth century, Nicolaus Copernicus concluded that the planets traveled around the
Sun. This was a very controversial idea, because it seemed to contradict the Bible and all the
accepted traditions of the time.

Galileo Galilei

In 1610, Galileo Galilei used a new optical instrument -- known today as a telescope -- to look
at the night sky. He discovered several points of light close to the planet Jupiter. He assumed
they were stars, fixed in the heavens, but their strange alignment intrigued him. The next night
he looked again, and saw that they had changed position. The bright points were dancing
around Jupiter --- they were not acting like stars at all. Galileo realized he was seeing bodies in
orbit around Jupiter. Copernicus was right - the heavens do not revolve around Earth.

Originally, Galileo named Jupiter's four bright moons after his sponsor, the Medici family.
Later, the moons were called Io, Europa, Ganymede, and Callisto, after mythical persons who
were companions of the god Jupiter. In honor of Galileo, they are now called the
Galilean moons.